var days = prompt("How many days?");
var remain = parseInt(days) % 7;
if ((remain + 2) < 7)
    remain = remain + 2;
else
    remain %= 7; 

switch(remain) {
  case 1:
    alert("Monday");
    break;
  case 3:
    alert("Wednesday");
    break;
  case 4:
    alert("Thursday");
    break;
  case 5:
    alert("Friday");
    break;
  case 6:
    alert("Saturday");
    break;
  case 7:
    alert("Sunday");
    break;
  default:
    alert("Tuesday");
}